This invention relates to a mold material for use in the manufacture of sandmolds for manufacturing metal castings, and more particularly to a mold material of this kind which can be formed into a sandmold which is excellent in strength after exposure under a room temperature atmosphere as well as strength after pouring molten metal into the sandmold and requires no mold wash or a very small amount of mold wash as obtained by spraying or the like.
Sandmolds used for manufacturing metal castings (hereinafter merely called "sandmolds") are generally manufactured by two major methods, i.e. one using an organic binder for setting foundry sand having a coarse grain size of 325 mesh or less, such as silica sand, zircon sand and chromite sand (hereinafter merely called "sand"), and the other using an inorganic binder for setting the sand.
The method using organic binder includes a method in which phenol resin or furane resin is mixed as a binder into sand and is cured by a high-acidity curing agent such as sulfuric acid, phosphoric acid, p-toluenesulfonic acid, and xylenesulfonic acid to cause the sand to set, a method in which phenol resin, polyisocyanate, and a basic catalyst are mixed into the sand, whereby the basic catalyst reacts with the phenol resin and the polyisocyanate to form urethane whereby the sand is set by the urethanic chemical reaction, and a method in which oil-denatured alkyd resin, metallic salt naphthenate, and polyisocyanate are mixed into sand so that they react with each other to form urethane whereby the sand is set by the urethanic chemical reaction. On the other hand, the method using inorganic binder for setting the sand includes a method in which cement is mixed into the sand to set same into a sandmold (OJ Process), and a method in which a of CO.sub.2 gas is blown into the sand impregnated with sodium silicate to set the sand.
However, a sandmold manufactured by any of the above-mentioned conventional methods using organic binder generally does not exhibit satisfactory strength of the sandmold after pouring molten metal thereinto (hereinafter called "casting strength"). Further, when molten metal is poured into the sandmold, the organic binder burns to cause unbinding of sand particles, often resulting in that part of the molten metal infiltrates into inner walls of the sandmold. To prevent this infiltration of molten metal, inner walls of the sandmold to be in contact with molten metal have to be subjected to mold washing, i.e. coating, by painting or spraying, with a mold wash material mainly composed of carbon graphite, mica powder, charcoal powder, or talcum powder. On the other hand, a sandmold obtained by any of the above-mentioned methods using inorganic binder is free of molten metal infiltration as mentioned above, but the sandmold is generally inferior in strength after being exposed under a room temperature atmosphere for some time period (hereinafter called "shelf strength") and often suffers from seizure, i.e. metal is stuck to inner walls of the sandmold. To prevent such seizure, it is necessary to add charcoal powder, coke powder, etc. into the sand, and then subject the inner walls of the resulting sandmold to mold washing. Thus, both of the two major methods require mold washing, of which the operation generally incurs about 30-50 percent of the total cost for manufacturing a sandmold, constituting a major factor for an increase in the manufacturing cost of sandmolds.